Pressed explosives are the explosives of choice for oilfield perforators. Pressing the explosive allows high volume production of precision shaped charges whereas casting or molding, the other two common production methods, do not. Critical to the pressing operation is the explosive molding powder itself. To date the vast majority of oilfield perforating charges are manufactured using wax coated PETN, RDX and HMX based explosives. The wax coating enables the explosive crystals to form into a pellet with adequate physical strength and dimensional stability when the formulation is pressed. The wax also serves to “desensitize” the explosive particles making them less prone to initiation when exposed to friction or impact.
In recent years the military has developed many pressed explosives with polymeric binders, so called “plastic bonded explosives” (PBX). PBX formulations offer several advantages over the standard “waxed” explosives. In general a PBX explosive formulation is made up of agglomerates of explosive crystals stuck together with polymer. “Waxed” formulations more closely resemble individually coated explosive crystals with little agglomeration. Therefore the flow, handling and molding properties of a PBX are typically much superior to those of a “waxed” explosive. The pressed PBX pellet is also much stronger, and is less sensitive to friction and impact.
The use of PBX explosives has been mostly limited to military and aerospace applications. The primary reason for this is that the typical polymeric binders used by the military do not have sufficient thermal stability for use in the oilfield environment. For example PBX's made with Ethylene Vinyl Acetate (EVA), Estane (a polyurethane) and Cellulose Acetate Butylrate (CAB) are important militarily, but are not useful at oil well temperatures because they undergo accelerated decomposition at higher temperatures. Further, most PBX formulations require heating the powder to elevated temperatures to soften the binder to allow compaction to high density.
Other reasons that polymeric binders have not been used in the oilfield market are time dependent stress relaxation of the polymers, which causes dimensional changes in the pressed charge over time, and liner cracking during the pressing process. As a result of these two complications a two-step shaped-charge loading process is typically used with PBX based explosives. The explosive formulation is first pressed to shape at a high density and then the shaped charge liner is inserted.